A two-dimensional capacitive touch panel includes a plurality of X-traces and a plurality of Y-traces mutually intersected to form a plurality of sensors. When an electrically conductive article such as a finger approaches a sensor, the coupling capacitor between the finger and the sensor will increase the capacitance on the trace connected to the sensor, so a detector circuit can locate the finger on the capacitive touch panel by detecting the capacitance variations of the traces, and then convert the position coordinates into information for a host to perform corresponding operation.
In further detail, as shown in FIG. 1, when a finger touches the intersection point between a Y-trace Y1 and X-traces X4 and X5, the measured capacitances from those X-traces X1, X2, X7 and X8 that are far from the finger will not be affected by the finger and remain below a threshold CTH, the measured capacitances from those X-traces X3 and X6 near the finger will be increased to slightly higher than the threshold CTH, the measured capacitance from the X-trace X5 is even higher, and the measured capacitance from the X-trace X4 where the finger touches is at the peak of the curve 10 established by the measured capacitances from the X-traces X1-X8. Similarly, the measured capacitance from the Y-trace Y1 is at the peak of the curve established by the measured capacitances from the Y-traces, since the Y-trace Y1 is touched by the finger. Therefore, the capacitive touch panel can identify the touch point (X4,Y1). The capacitances on the X-traces X1, X2, X7 and X8 are not affected by the finger, so the measured capacitances therefrom are too low to be useful in the subsequent conversion and calculation for identification of the touch point.
In actual circuitry the capacitance can not be detected directly, and thus a capacitive touch panel performs the capacitance detection by charging its traces one by one and detecting the voltage variations on each of the traces to extract the capacitance variations of the traces. After conversion and processing, the position information is obtained. FIG. 2 is a circuit diagram of a conventional successive approximation register (SAR) analog-to-digital converter (ADC), in which a sample-and-hold circuit 12 samples the trace voltage Vin of a sensed trace to generate a sampled voltage Vs as an input of a comparator 14, a SAR and control circuit 18 provides a successive approximation digital code D0-Dn−1 for a digital-to-analog converter (DAC) 16, with reference to a reference voltage Vref, to convert into a voltage VD as another input of the comparator 14, the comparator 14 compares the voltage VD with the sampled voltage VS to generate a comparison result for the SAR and control circuit 18, the SAR and control circuit 18 adjusts the successive approximation digital code D0-Dn−1 according to the comparison result, another voltage VD is thus generated by the DAC 16 and compared with the sampled voltage VS, the foregoing procedure is repeated so as for the voltage VD to approximate the sampled voltage VS step by step, and finally the digital code D0-Dn−1 generated is supplied to a firmware to fill in a frame as the measured capacitance of the sensed trace.
The conventional detect methods always sense all the traces one by one with the process that charges a sensed trace and detects the trace voltage therefrom, and performs the aforesaid successive approximation and conversion to generate a digital code for this sensed trace for a firmware. After this trace is sensed, the same process is repeated to the next trace. When the digital codes of all the traces are obtained, they are used to construct a frame of information to position a touch point. Since the digital code converted from the trace voltage of a trace without significant capacitance variation is useless, the present invention proposes a method for pre-filtering those useless traces to reduce the operation of the detector circuit, thereby either increasing the frame speed or reducing the power consumption, to improve the performance of a capacitive touch panel.